With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries used in large size applications which have excellent storage characteristics. Under these circumstances, the lithium ion secondary batteries having advantages such as large charge/discharge capacities have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of large charge/discharge capacities thereof. However, these materials tend to be deteriorated in thermal stability upon charging and charge/discharge cycle durability, and, therefore, it has been required to further improve properties thereof.
Specifically, when lithium is released from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li released reaches 0.45, the crystal structure of such a lithium-released region of LiNiO2 is transformed from a hexagonal system into a monoclinic system, and a further release of lithium therefrom causes transformation of the crystal structure from a monoclinic system into a hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary batteries tend to be deteriorated in cycle characteristics or suffer from occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage characteristics of the secondary batteries. To solve these problems, various studies have been made on materials to which Co and Al to are added by substituting a part of Ni in LiNiO2 therewith. However, these materials have still failed to solve the above-described conventional problems. Therefore, it has still been required to provide a composite oxide having a higher stability.
Hitherto, in order to improve various properties of LiNiO2 particles such as stability of a crystal structure, charge/discharge cycle characteristics and thermal stability, various methods have been attempted. For example, there are known the technique in which the surface of LiNiAlO2 is coated with an Li—Ni—Co—Mn composite oxide to improve cycle characteristics thereof (Patent Document 1); the technique in which different kinds of materials, i.e., an Li—Co composite oxide and an Li—Ni—Co—Mn composite oxide are mixed with each other to improve charge/discharge cycle characteristics of the Li—Co composite oxide (Patent Document 2); the technique in which lithium carbonate, Ni(OH)2, Co(OH)2 and manganese carbonate are suspended in an Li—Co composite oxide, or the Li—Co composite oxide is mechanically treated and coated with an Li—Ni—Co—Mn composite oxide, to improve charge/discharge cycle characteristics of the Li—Co composite oxide (Patent Document 3 and Patent Document 4); the technique for improving a crystallinity or a thermal stability of composite oxide particles by coating the particles with a fluorine compound (Patent Documents 7 and 8); and the like. However, these conventional techniques have been still insufficient to improve the properties of the positive electrode active substance particles.
In recent years, it has been found that a positive electrode active substance comprising Li2MnO3 belonging to a space group of C2/m and having a higher capacity exhibits large charge/discharge capacities. However, it is known that a secondary battery produced using such a positive electrode active substance must be charged at a high potential, and therefore tends to have fatal disadvantages, i.e., tends to be deteriorated in cycle characteristics (Patent Document 5). There has been reported the technique using the above positive electrode active substance which is improved in cycle characteristics. However, the technique tends to be still insufficient in improvement of the cycle characteristics (Patent Document 6).
In addition, from the reasons as described above, as the voltage at which a battery is subjected to charging is increased, there occurs such a tendency that a positive electrode active substance used in the battery becomes more unstable owing to accelerated oxidation of a transition metal contained therein. The positive electrode active substance comprising Li2MnO3 belonging to a space group of C2/m is to be subjected to charging at a high voltage, and therefore is required to have a higher thermal stability.